I use long rulers (3 to 4 feet long) or curtain rods as light weights to keep my backing smooth.
I center the backing on the table, and then pin up three to four inches of the backing that is hanging down on each side and slide a long ruler or curtain rod into the "temporary rod pockets" - that give a slight tension to the backing that keeps it smooth - but not so much that it stretches the backing (very much). I might put a couple of weights on the top of the table to keep this from sliding until I get the batting and top placed.
Then I center the batting and top of this. I found folding the batting and top into fourths works - and then I line up the "center points" when layering.
Then I start pinning from the center and work my way to the edges. I do put weights on the top to keep things from sliding off the table. I slide the sandwich around until I am done.